Various types of mechanisms have been developed for counterbalancing the weight of upward acting doors, particularly sectional garage doors, so as to minimize the effort of opening and closing the door and to minimize the power requirements of power operated door opening and closing mechanisms. One widely used type of counterbalance mechanism comprises, generally, spaced apart cable drums mounted on a shaft supported on a wall adjacent to the door opening and connected to flexible cables which, in turn, are connected to lower opposed edges of the door. Typically, one or more torsion springs are sleeved over the shaft, keyed to the shaft for exerting a torsional effort thereon and on the cable drums. One end of the spring or springs must, of course, be anchored or stationary with respect to the shaft to provide for the torsional windup and unwinding which occurs during closing and opening movement of the door and during adjustment of the springs to provide a suitable counterbalance effect.
Several problems have persisted in the development of suitable counterbalance mechanisms for upward acting doors of the general type discussed herein. There is the ever present need to reduce the cost and maintain the reliability and ease of operation of such mechanisms, particularly, counterbalance mechanisms which require multiple torsion springs for larger doors. Moreover, various arrangements of conventional spring attachment mechanisms comprising collars connected to one end of the springs and locked to the cable drum shaft with setscrews have been a cumbersome and inaccurate arrangement for setting and adjusting the counterbalance torque of the spring or springs. Various types of spring adjustment mechanisms have been developed, including mechanisms which utilize a worm gear drive, in an effort to overcome the problems associated with collar-type spring anchors and adjustment devices. However, prior art counterbalance spring adjustment mechanisms have been difficult to use, particularly if they are centered or disposed substantially inboard of the spaced apart support brackets for the counterbalance cable drums.
Still further, prior art counterbalance mechanisms with worm gear drive adjustment devices have been somewhat complex, expensive to manufacture, difficult to operate and service and have otherwise been plagued with shortcomings which have made it desirable to seek further improvements in counterbalance systems for vertical opening doors, including sectional garage doors used in commercial and residential buildings and garages.